Séminaire
The correct simulation of midlatitude atmospheric blocking has always been a main concern since the earliest days of numerical modeling of Earth’s atmosphere. We here assess 20 years of global climate model (GCM) developments by comparing with a common metric simulations from the AMIP1 (1992), the CMIP3 (2007), and the CMIP5 (2012) intercomparison projects. Although large improvements are seen over the Pacific Ocean, only minor advancements have been achieved over the Euro-Atlantic sector. Some of the most recent GCMs still exhibit the same negative bias as 20 years ago, associated with large geopotential height systematic errors. Some individual models, nevertheless, have improved and do show good performances in both sectors. Since increased horizontal resolution seems able to alleviate the Euro-Atlantic blocking bias, we thus analyze a set of atmosphere-only climate simulations produced with the EC-Earth GCM at five different horizontal resolutions (from 125 km to 16 km). Results show that the usual negative bias in blocking frequency over Europe becomes negligible at 40-km and 25-km resolution. A combined effect by the more resolved orography and by a change in tropical precipitation is identified as the source of an upper tropospheric planetary wave. At the same time, a weakening of the meridional temperature gradient reduces the upper level baroclinicity and the zonal mean winds. Following these changes, the high resolution configurations show a weakened Atlantic eddy-driven jet stream favoring the breaking of synoptic Rossby waves over the Atlantic ridge and thus increasing the simulated European blocking frequency. However, at high-resolution the simulated Atlantic jet stream is too weak and the blocking duration is still underestimated, suggesting that the optimal blocking frequencies are achieved through compensation of errors.
bruno.deremble@ens.fr